It is known that polyisocyanates can be used as additives for adhesives based on polymers dissolved in organic solvents. Solutions of natural or synthetic rubber or solutions of polyurethane-polyfunctional polyisocyanates have been used for some time to obtain better adhesion to numerous materials to be bonded, greater stability of the bond at elevated temperatures and greater resistance to chemicals. It has now surprisingly been found that it is possible to improve the bonding properties of aqueous adhesives based on water-dispersible polymers by the addition of certain hydrophilically-modified polyisocyanates described herein.
This discovery is surprising because it had been expected that the isocyanate groups of hydrophilically-modified polyisocyanates would react with the water present to form polyureas without influencing the bonding properties of the dispersed polymer. It was also particularly surprising to fluid that, even in the case of adhesive dispersions based on polymers containing very few, if any, active hydrogen atoms, the use of polyisocyanate preparations according to the invention leads to a significant improvement in the bonding properties of these adhesives.